Semiconductor devices are active element-including electronic devices that utilize electrical characteristics of a semiconductor. Such semiconductor devices have been widely applied to audio-visual apparatuses, communication facilities, computers, and electrical household appliances. Among them, semiconductor devices including a three-terminal active element such as a thin film transistor (also referred to as “TFT”) have been widely used in various fields. Such semiconductor devices have been used as a switching element, a control circuit and the like in a display device such as an active matrix liquid crystal display device (herein after, also referred to as “liquid crystal display”).
A reduction in power consumption has been strongly needed for such TFT-including semiconductor devices, which is used in liquid crystal displays and the like, recently. Also for the TFT, a reduction in power consumption has been needed. To finely form each member of the TFT may be mentioned as a method of reducing power consumption of the TFT. It is essential to reduce a threshold voltage (Vth) of the TFT by thinning a gate insulating film disposed between a semiconductor layer and a gate electrode. A silicon oxide film excellent in interface characteristics is generally used as the gate insulating film. A low threshold operation of the TFT is permitted if the gate insulating film having a single layer structure consisting of a silicon oxide film is formed to have a small thickness. However, in this case, a breakdown voltage of the gate insulating film is reduced and defects such as an increase in leakage current between the semiconductor layer and the gate electrode tend to be generated.
Polysilicon (herein after, also referred to as “p-Si”) has been recently used as the semiconductor layer of the TFT used in a liquid crystal display, which realizes a liquid crystal display in which peripheral driver circuits such as a driving circuit and a control circuit, and a pixel part are integrally formed on a substrate, a so-called monolithic liquid crystal display. As a method of forming p-Si on the substrate, a method of crystallizing amorphous silicon (herein after, also referred to as “a-Si”) by laser irradiation, a so-called laser annealing method has been commonly used. However, in this method, a large projection is generated on the p-Si layer surface, and therefore if the gate insulating film is thinned, coverage characteristics of the gate insulating film for the p-Si layer (herein after, also referred to as “coverage of the gate insulating film”) is particularly deteriorated. Electric field concentration often occurs at the end of the projection, which further reduces the breakdown voltage. As a result, defects such as insulating breakdown and leakage current tend to be caused in TFTs including polysilicon as a semiconductor layer (herein after, also referred to as “p-TFT”). Accordingly, a liquid crystal display including a p-TFT as a switching element has room for improvement because defects such as point defects in a panel at an initial state and reliability defects of the device are increased, thereby reducing the yield.
For this problem, a technology in which a polycrystal silicon thin film is isotropically etched using plasma gas to be flattened is disclosed (for example, refer to Patent Document 1). A technology in which recrystallization energy is provided for the polycrystal silicon layer by laser irradiation and the like to form a flattened polycrystal silicon layer is disclosed (for example, refer to Patent Document 2). Further, a technology in which an amorphous semiconductor film (amorphous silicon thin film) formed on the first thin film (silicon oxide film) and the second thin film is heated by laser to form a uniform and flattened polycrystal semiconductor film (polycrystal silicon thin film) having a large crystal particle diameter is disclosed (for example, refer to Patent Document 3). According to these, the p-Si layer is flattened, and therefore, defects such as insulating breakdown and leakage current may be suppressed. However, the effect of suppressing a reduction in breakdown voltage when the gate insulating film is thinned is insufficient.
Accordingly, the semiconductor device including a flattened polycrystal semiconductor such as polysilicon in a semiconductor layer has room in order to prevent a reduction in withstand voltage even if the gate insulating film is thinned.
[Patent Document 1]
    Japanese Kokai Publication No. 2000-133634[Patent Document 2]    Japanese Kokai Publication No. 2001-60551[Patent Document 3]    Japanese Kokai Publication No. 2001-127302